1. Field of the Invention
The present invention relates to a mobile communication system constituted by a switching station, base stations connected to the switching station, and a plurality of mobile stations moving in a service area formed by communication zones due to the base stations.
2. Description of the Prior Art
FIG. 1 is a system constitution diagram showing a mobile communication system in the prior art disclosed, for example, in Japanese patent application laid-open No. 38940/1985. In FIG. 1, the mobile communication system comprises a switching station 1, base stations 2a-2j connected to the switching station 1, a wired network 3 connecting between the switching station 1 and the base stations 2a-2j, communication zones 4a-4j of the base stations 2a-2j, a crossover zone 5 where the communication zones 4a-4j intersect each other, and a mobile station 6 moving in a service area 7 formed by the communication zones 4a-4j.
The base stations 2a-2j and the mobile stations 6 have a speech radio channel (hereinafter referred to as "S-ch") for speech, and a control radio channel (hereinafter referred to as "C-ch") for communication of communication control signals. These channels S-ch and C-ch are constituted by an upward channel from the mobile station 6 to the base stations 2a-2j, and a downward channel from the base stations 2a-2j to the mobile station 6 respectively.
Next, operation will be described. FIG. 2 is a timing chart showing signals transmitted or received between the base stations 2a-2j and the mobile stations 6 in the mobile communication system in such constitution. The operation will be described in an example of position registration as follows.
When the position information of the mobile station 6 becomes necessary, the base stations 2a-2j transmit the position registration request signals (a) continuously using the downward channel of C-ch. The mobile station 6 at non-speaking state waits the position registration request signals (a) from the base stations 2a-2j. When the position registration request signal (a) is received in the communication zone where the mobile station 6 is located at present, for example, in the downward channel of C-ch from the base station 2a of the communication zone 4a, if the base station identification code included in the position registration information signal (a) is different from that stored by the mobile station 6 at the last position registration before then, the downward channel of S-ch with an empty signal (b) transmitted from the base station 2a is detected.
When the mobile station 6 detects a signal of the downward channel of S-ch with the empty signal (b) transmitted, the mobile station 6 transmits a position registration signal (c) in the upward channel of S-ch corresponding thereto. The switching station 1 performs the position registration based on the position information. From taking synchronization of the position registration signal (c) until finishing its reception, the base station 2a transmits a transmission inhibiting signal (d) to the downward channel corresponding to the upward channel of S-ch where the position registration signal (c) is transmitted. Thereby the transmission of the position registration signal (c) to the same upward channel from other mobile station 6 can be prevented.
In the mobile communication system in the prior art constituted as above described, since only one base station 2a-2j is installed at the same communication zone 4a-4j, even if the emergency state occurs during use of the channel S-ch by the base station 2a-2j, the general communication cannot be performed from the mobile station 6 within the corresponding communication zone 4a-4j. Also even if the mobile station 6 intends to perform the general communication using the channel C-ch, the general communication cannot be normally performed due to collision of the channel C-ch with other mobile station or other base station.
When the communication zones 4a-4j are constituted simply by a plurality of base stations, the collision of C-ch of these stations causes the communication failure. Even if the time difference is taken for the transmission of C-ch in order to avoid such collision of C-ch, since the base station identification code is different per each base station, the mobile station 6 even in the same communication zone responds to the position registration request signal from each base station of the communication zone. Also the mobile station 6 storing the base station identification code of the base station of some communication zone waits signals in the communication processing including the base station identification code and does not respond to signals in the same communication processing including the base station identification code from other base station in the same communication zone thereby a long time is required until the execution of the communication processing.
FIG. 3 is a system constitution diagram of another mobile communication system in the prior art disclosed, for example, in Japanese patent application laid-open No. 147903/1977. In FIG. 3, the mobile communication system comprises a switching station 1, base stations 2a, 2b connected to the switching station 1, a telephone terminal 20 incorporated in the switching station 1, communication zones 4a, 4b of the base stations 2a, 2b, a mobile station 6 moving in a service area formed by the communication zones 4a, 4b, wired networks 3a, 3b connecting the base stations 2a, 2b to the switching station 1, information of signal-to-noise ratio-(hereinafter referred to as "SN ratio") 17a, 17b transmitted from the base station 2a or 2b to the switching station 1, base station control signals 18a, 18b for the switching station 1 to control the base stations 2a, 2b, and speech paths 19a, 19b installed in the switching station 1 for connecting the base stations 2a, 2b to the telephone terminal 20.
The base stations 2a, 2b and the mobile station 6 have a speech radio channel (hereinafter referred to as "S-ch") for speech, and a control radio channel (hereinafter referred to as "C-ch") for communication of communication control signals. These channels S-ch and C-ch are constituted by an upward channel from the mobile station 6 to the base stations 2a, 2b, and a downward channel from the base stations 2a, 2b to the mobile stations 2a, 2b to the mobile station 6 respectively.
Next, operation will be described. Assume now that speech exists between the telephone terminal 20 and the mobile station 6 in the communication zone 4b through the base station 2b.
Then the switching station 1 detects the movement of the mobile station 6 in following manner. That is, the base station 2b always measures the SN ratio of signals from the mobile station 6, and if the measured value is the standard value or more, the SN ratio information 17b is transmitted, for example, as "1" to the switching station 1. If the mobile station 6 is moved to the communication zone 4a, the measured value of the SN ratio in the base station 2b becomes the standard value or less. Consequently, the SN ratio information 17b from the base station 2b becomes "0" for example.
Since the SN ratio information 17b becomes "0", the switching station 1 detects the movement of the mobile station 6 to the outside of the communication zone of the base station 2b, and executes changing of the channel during speech. The switching station transmits the control signal 18a to the base station 2a adjacent to the communication zone, and commands the reception of the channel S-ch with frequency used by the mobile station 6 in the communication zone 4b. The base station 2a receives the channel S-ch, and if the SN ratio is the standard value or more, the SN ratio information 17a is transmitted as "1" to the switching station 1. On receiving the information, the switching station 1 changes the speech path 19a into the speech path 19b, and realizes the speech in the mobile station 6--the base station 2a--the switching station 1--the telephone terminal 20, and restores the base station 2b.
Since other mobile communication system in the prior art is constituted as above described, the switching station 1 must always measure and monitor the SN ratio of the received signals from the mobile station 6 regarding all base stations 2a, 2b. Also when the movement of the mobile station between the communication zones 4a, 4b is detected, the SN ratio information must be collected regarding all base stations at the periphery of the communication zone 4b before the movement simultaneously or in sequence, in matching with the channel S-ch of frequency used by the mobile station 6 and the base station 2b within the communication zone 4b before the movement, until the communication zone 4a of the movement destination can be found. Consequently, problems exist in that large burden is applied to the processing.